This invention is related to commonly owned U.S. Pat. No. 4,579,107, the application of which is co-pending herewith and incorporated herein.
The present invention relates to solar collectors and methods of producing the same, but more specifically to a low cost process for making an absorber plate, selective surface and/or the exterior member of the frame.
Solar engineers have continually struggled to compromise quality and performance for price. Eighty to ninety percent of the manufacturing cost of solar collectors is usually in materials, with the absorber plate being the most expensive component. Hence, cutting the cost of the absorber plate has been the primary goal of most manufacturers. Designs are now being marketed where the number of tubes in the absorber plate has been significantly reduced, said tubes being attached mechanically or bonded with adhesives to the absorber plate resulting in a serious loss of efficiency. U.S. Pats. to Boyd (4,074,406), Andrassy (4,089,326), Heinemann (4,245,620), Bleckmann (4,369,836) and Grahman (4,517,721) show typical absorber plates with tubes mechanically fastened or cemented into place. Mechanically fastened tubes lack adequate thermal contact with the absorber plate, decreasing heat flow, and hence, lowering efficiency. Small gaps created between the absorber tube and plate leads to the problem of corrosion which seriously impair heat flow, not to mention deterioration of the absorber plate. Adhesives are sometimes used in an attempt to overcome these problems, but are often unsuccessful. Adhesives have very limited life spans under conditions of high heat, humidity and ultraviolet light as those experienced in a solar collector. In addition, adhesives bond extremely poorly to many metals such as copper, a highly desirable metal for making absorber plates.
In order to further cut costs, manufacturers have been switching from copper to other metals, such as aluminum, as the base material for the collector absorber plate, retaining copper absorber tubes. In doing so, designers are limited to the above techniques to fasten absorber tubing to the plate, as copper cannot be soldered, brazed or welded to aluminum by traditional methods.
The present invention provides an easy solution to the problem by efficiently allowing one to permanently embed tubing of almost any type of material including copper directly within an absorber of almost any metal including aluminum, while eliminating the possibility of any galvanic corrosion, a serious drawback to many of the previously mentioned designs. Limited success in fastening copper and aluminum has been met by some manufacturers using high frequency resistance welding and/or mechanical compression, however, the processes are very labor intensive and require expensive equipment. Mechanical bonds are unreliable and high frequency bonds allow only a single tube at a time to be fixed to the absorber plate. Thus, manufacturers limit the number of tubes used, which seriously compromises efficiency.
The method of the present invention enables formation of the absorber plate around the absorber tubes regardless of their number with the same amount of effort, energy, time and equipment. This technique avoids the use of manifolds to connect respective tubes, unlike the welding and mechanical compression techniques of the prior art. A sinusoidal shape tube pattern including the bends would be very difficult or impossible to manufacture according to present state of the art fabrication techniques. In contrast, in the present invention, the absorber plate metal is laid down in a liquid state where it conforms to a substrate and the tubes can be arranged in any geometric shape. In addition, the present invention precisely vary the thickness of the absorber plate in predetermined areas, for maximum heat transfer. Moreover, manufacturers of solar absorber plates have traditionally used selective surfaces to increase the efficiency of the absorber plate, a selective surface being a coating on that surface of the collector exposed to the sun, that absorbs the maximum amount of solar radiation while emitting a minimum amount of black body radiation. The most common selective surfaces with the highest performance are black nickel and black chrome, both applied by electroplating. Although very efficient, they are very expensive processes, representing a substantial share of the cost of the absorber and hence the entire collector cost. The process involves the use of a very complicated series of cleaning, chemical, etching, and electroplating tanks, being very labor and capital intensive. The present invention significantly lowers the cost of the selective surface by applying the appropriate metals in the same manner using the same equipment as the process used to form the absorber plate. Consequently, the absorber needs only to be dipped in a single chemical conversion bath resulting in a high quality selective surface.
Additionally, absorber plates may be formed from metallic foil or sheets that already have the selective surface plated on. The present process allows one to form the absorber tubes as-part of the absorber plate without overheating and damaging the existing selective surface during the manufacturing process. The advantage of this technique is that it is much more cost effective to buy large rolls or coils of foil or sheet with the selective surface already plated on it, than to make each individual absorber and electroplate the selective surface on separately. Any attempts to weld, braze or solder the tubes to the sheet or foil according to traditional methods would seriously damage the existing selective surface. Mechanical compression and high frequency welding would also be extremely damaging to the existing selective surface.
The present invention further facilitates the process used in forming the selective surface and the absorber plate to form the outer layer of metal for the collector frame and bottom of the collector. Hence, the same equipment can do almost all operations to form the collector all at the same time, streamlining production and capital equipment costs.
Traditionally solar collector manufacturers have insulated the absorber plate and/or collector with fiberglass or a foamed insulation product such as styrofoam, polyurethane or polyisocyanurate. Because of the low insulating value of fiberglass insulation, the base and walls of the collector are very thick to provide proper insulating value, and hence impractical. On the other hand, foamed products as those mentioned above cannot support the elevated operating temperatures experienced inside the collector, which is required for certification. Usually a combination of the two is used, with a layer of fiberglass separating the foam from the absorber plate. If any moisture enters the collector, which eventually happens in almost all collectors, the fiberglass becomes waterlogged, which significantly lowers the efficiency of the collector. The present invention incorporates a wider choice of materials to insulate the absorber and/or collector, in particular those that are much cheaper, can easily withstand stagnation temperatures, and if desired, form the body or structure for the collector. Hence, if the entire collector is formed from a single, unibody construction, the number and size of joints is significantly reduced or eliminated, thus greatly reducing the probability of leaks.
U.S. Pat. No. to Barrett (4,244,356) discloses a method of using a unibody frame molded out of fiberglass or ABS plastic and filled with insulation. Neither the fiberglass or ABS frame will survive the conditions experienced in the collector in the vicinity of the absorber plate. Extreme conditions of overheating, moisture and ultraviolet light degradation exist. The present invention is a vast improvement in that it can use a wider variety of materials as the structural support or body of the collector while incorporating a skin applied directly to it that can easily withstand the hostile environment inside the collector.